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Bronze Age origins of bubonic plague

Three pandemics of bubonic plague have occurred in historical times: the first began with Plague of Justinian from AD 541 to 544, continuing intermittently until AD 750 AD; the second began with the Black Death from AD 1347 to 1351, continuing in waves including the Plague of 1665-66 into the eighteenth century; and the third which started in China in the mid-nineteenth century and triggered a series of outbreaks worldwide during the first half of the last century. The Black Death alone killed 30 to 50 percent of the European population. Deaths totalled at least 75 million, more than the number of deaths during World War I and II combined.

The cause of this deadly disease was identified as the flea-borne bacterium Yersinia pestis in 1894 by Swiss biologist Alexandre Yersin. More recently, genetic studies have suggested that it diverged from the more widespread but less virulent Yersinia pseudotuberculosis anywhere between 2,600 and 28,000 years ago.

In a newly-published study, researchers investigated the origins of Y. pestis by sequencing ancient bacterial genomes obtained from the teeth of Bronze Age people across Eurasia dating from 2,800 to 5,000 years ago. Their findings indicate that the flea-borne strain that caused the historic period plague pandemics evolved from a less virulent strain that was present in human populations long before any records of plague outbreaks.

The strains infecting Bronze Age Eurasian populations lacked the Yersinia murine toxin (ymt) gene, which encodes a phospholipase D protecting the bacterium inside the flea gut, so enabling fleas to act as vectors. Similarly, mutations associated with the development of bubonic plague and evading mammalian immune systems had not yet occurred. Not until around 3,000 years ago did highly virulent, flea-borne strains emerge.

The researchers also estimated the divergence from Y. pseudotuberculosis at 55,000 years ago, twice as early as previous maximum estimates. The Bronze Age strains began to diverge from one another 5,800 years ago. Although they could not cause bubonic plague, they could still cause pneumonic and septicemic plague and these might have been responsible for population declines between the late fourth and early third millennium BC. Large scale population movements and social changes during the Bronze Age might have facilitated plague outbreaks, but not on the scale of the historical era flea-borne pandemics.